The possible roles of a 14-kDa human thioredoxin (Trx)-related protein (TRP14) in TNF-α signaling were studied in comparison with those of Trx1 by RNA interference in HeLa cells. Depletion of TRP14 augmented the TNF-α-induced phosphorylation and degradation of IκBα as well as the consequent activation of NF-κB to a greater extent than did Trx1 depletion. Deficiency of TRP14 or Trx1 enhanced TNF-α-induced activation of caspases and subsequent apoptosis by a similar extent. The TNF-α-induced activation of c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinases (MAPKs), however, was promoted by depletion of TRP14 but not by that of Trx1. Unlike Trx1, TRP14 neither associated with nor inhibited the kinase activity of apoptosis signal-regulating kinase-1 (ASK1), an upstream activator of JNK and p38. In combination with the results in the accompanying paper that TRP14 did not reduce the known substrates of Trx1, these results suggest that TRP14 modulates TNF-α signaling pathways, provably by interacting with proteins distinct from the targets of Trx1. In an effort to identify target proteins of TRP14, a mutant of TRP14, in which the active site cysteine (Cys46) was substituted with serine, was shown to form a disulfide-linked complex with LC8 cytoplasmic dynein light chain. The complex was detected in HeLa cells treated with H2O2 or TNF-α but not in untreated cells, suggesting that LC8 cytoplasmic dynein light chain is a possible substrate of TRP14.